1. Field of the Invention
The present invention relates generally to laser diodes and, more particularly, to laser diode drivers.
2. Description of the Related Art
The modern field of lightwave communications has greatly enhanced information transmission. Optical fibers are the preferred transmission path of this dynamic field and semiconductor laser diodes are the preferred generators of coherent light which is transmitted through this transmission path. In an exemplary process of this field, communication data is converted to current pulses and, in response to the current pulses, laser diodes emit coherent light pulses which are guided over long distances through low-loss optical fibers.
A laser diode typically has a threshold current ithld (e.g., 20-30 milliamps) which is the minimum current at which the diode lases. After lasing is initiated, the laser diode""s light emission becomes much more efficient and is substantially proportional to additional diode current. Accordingly, laser diode current pulses are typically formed with a constant bias current ibias which is just above the threshold current ithld and an added modulation current imod that is responsive to a data signal. Because both current levels exceed the threshold current ithld, they cause a laser diode to generate coherent light with power levels that respectively correspond to the bias current ibias and to the sum of the bias current ibias and the modulation current imod.
The graph 20 of FIG. 1 illustrates an exemplary plot 22 of a laser diode""s optical power as a function of diode current. Optical power is low and substantially constant until the diode current reaches the threshold current ithld. Above the threshold current ithld, the diode is in its lasing mode and output power is substantially proportional to the diode current.
FIG. 1 illustrates current pulses 24 whose lower and upper current levels are respectively the bias current ibias and the sum of the bias current ibias and the modulation current imod. The substantially-linear portion of the plot 22 shows that a laser diode will respond to the current pulses 24 by generating light pulses 26 which have a lower power P0 and an upper power P1 that respectively correspond to the lower and upper current levels of the current pulses 24.
Various laser diode drivers have been proposed for converting data pulses into laser diode current pulses. These conventional drivers typically provide a constant bias current ibias which is driven through the laser diode and a differential pair of transistors which respond to data pulses by steering a current pulse of magnitude imod through the laser diode. The modulation current imod is generally generated by a current mirror which is coupled as a tail current to the emitters of the differential pair. The current pulses are typically coupled through a capacitor to the laser diode and an inductor is coupled in parallel across the laser diode and the capacitor.
These conventional laser drivers are preferably realized as monolithic integrated circuit chips to reduce their size and cost. Their structure, however, degrades the possible advantages of such chips. For example, the bias voltage VCC of the chips is preferably limited to 5 volts which does not provide sufficient headroom for collectors of the differential pair. In order to resolve this problem, the differential pair is generally coupled to the laser diode with the capacitor/inductor network described above but as a consequence, settling effects in the capacitor/inductor network generate timing inaccuracies when the laser diode is driven at different duty cycles. Increasing the values of the inductor and capacitor reduces these inaccuracies but this requires that these elements be located adjacent the chips which increases driver complexity, size and cost.
The present invention is directed to laser diode drivers which are directly coupled so as to eliminate coupling elements that introduce timing inaccuracies and add complexity, size and cost. In addition, these drivers reduce power dissipation and maintain sufficient transistor headroom to enhance response time and dynamic range.
A laser diode driver of the invention includes a data switch that responds to data pulses by driving current pulses through a laser diode to generate corresponding light pulses wherein the data pulses are defined by first and second pulse levels. A data switch embodiment includes:
a) a bias current mirror that drives a bias current ibias through the laser diode in response to an input current;
b) a modulation current source that generates an imod/n current,
c) a modulation current mirror connected to receive the imod/n current and, in response, drive a modulation current imod through the laser diode, and
d) a modulation differential pair of transistors that steers the imod/n current away from the modulation current mirror in response to the first pulse levels.
Accordingly, the modulation current mirror drives imod current pulses through the laser diode during the second pulse levels with no need for coupling elements.
The laser diode driver further includes a controller which receives a feedback light signal from the laser diode and, in response, provides the input current to the bias current mirror and a tail current to the modulation differential pair that are varied to stabilize the laser diode""s light pulses.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.